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Foreign object damage and fatigue crack threshold



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International Journal of Fracture 107 31 51 2001 2001 Kluwer Academic Publishers Printed in the Netherlands Foreign object damage and fatigue crack threshold Cracking outside shallow indents XI CHEN and JOHN W HUTCHINSON Division of engineering and applied sciences Harvard university Cambridge MA 02138 USA Received 23 November 1999 accepted in revised form 9 June 2000 Abstract Foreign Object Damage FOD usually happens when objects are ingested into jet engines powering military or civil aircraft Under extreme conditions FOD can lead to severe structural damage More commonly it produces local impacted sites of the fan and compressor airfoils lowering fatigue life of these components FOD is a prime cause for maintenance and repair in aircraft engines In this paper a framework for analyzing FOD and its effect on fatigue cracking is established A finite element analysis is used to identify three relevant regimes of FOD related to the depth of penetration into the substrate and to determine the residual stresses Most of the emphasis in this paper focuses on fatigue cracks emerging from shallow indentations which are generally expected to be of most practical concern Full three dimensional finite element solutions are obtained for semicircular surface cracks emerging from specific locations at the indentation revealing the influence of the residual stress on the stress intensity factor distribution For shallow indents a relatively simple dimensionless formula for the relation between the residual stress intensity factor the crack size and the indentation width are developed These results together with results for the intensity factor variations due to cyclic loading have been used to address the question To what extent do the residual stresses caused by the FOD reduce the critical crack size associated with threshold fatigue crack growth Formulas for the critical crack size are obtained Specific results are presented for the blade alloy Ti 6Al 4V revealing that FOD can



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